Enterotoxin production in relation to taxonomic grouping and source of isolation of Aeromonas species

Abstract

A total of 19 of 20 (95%) strains of A. hydrophila biovar hydrophila and 16 of 17 (94%) strains of A. sobria isolated from a variety of clinical and environmental sources were found to be enterotoxin positive. Only 2 of 18 (11%) A. hydrophila biovar anaerogenes and 2 of 13 (15%) unidentified Aeromonas strains from a similar variety of sources produced enterotoxin. No association was apparent between the source of isolation, in particular diarrheal stools, and enterotoxigenicity; 41% of the isolates from diarrheal stools were enterotoxin negative. A strong correlation was noted between ability to produce enterotoxin and positive results in 6 characters: lysine decarboxylase and Voges-Proskauer reactions, production of gas from glucose, gluconate oxidation, xanthine hydrolysis and hemolysis of human erythrocytes. In the majority of cases (35 of 39 strains), enterotoxigenicity was detected using cell-free filtrates of brain heart infusion broth cultures grown in 36.degree. C for 15 h; the other 4 positive isolates were detected after growth in the same broth at 30.degree. C or in Casamino Acids-yeast extract broth at 30.degree. or 37.degree. C. It is recommended that for enterotoxin tests, strains should be grown in both media at both temperatures. The infant mouse test was found to be a simple and reliable method for detection of the enterotoxin. The toxin proved to be heat labile and not neutralized by cholera antitoxin.